From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758143Ab0CaUbs (ORCPT ); Wed, 31 Mar 2010 16:31:48 -0400 Received: from mail-yw0-f172.google.com ([209.85.211.172]:34205 "EHLO mail-yw0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757834Ab0CaUbq (ORCPT ); Wed, 31 Mar 2010 16:31:46 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:date:message-id:subject:from:to:cc:content-type; b=rmB3mpSpx+VIRQ8H5bK/BSSKkKkvZ9iELhRqyyHm9DeZ8hXCHqDyEcvXnFNCr3L5/i DVqhbqEqgWzE/qbgZpGOWu1m1Vl8Ej8RwyNaDoKUCjwkH7FnMxfguREwjBJLbxDGmyO5 4q0J/UP0hTrz7U5uxLiHVJEth1sR+/npyJd3s= MIME-Version: 1.0 Date: Wed, 31 Mar 2010 16:24:09 -0400 Message-ID: Subject: Questions about RAID and I/O scheduler From: Yuehai Xu To: linux-raid@vger.kernel.org Cc: linux-kernel@vger.kernel.org, yhxu@wayne.edu, czoccolo@gmail.com, neilb@suse.de, jens.axboe@oracle.com Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, I noticed that some one said NOOP is usually the default I/O scheduler for hardware RAID. Why not CFQ? Suppose there are just several sequential read processes, as I know CFQ will keep all the disk heads of hard raid to serve a process for a while(a time slice), in that case, CFQ should be the best of all I/O schedulers. Am I right? Generally, hard raid should have its own I/O scheduler in their firmware, in that case, the I/O scheduler of OS should do nothing except dispatch the requests as soon as possible, it is the hard raid itself to decide how to schedule these requests. From this point of view, NOOP should be the default one. I am really confused here. The next question is about the maximal number of disks in disk array, the fault tolerance should be one limitation because the more the number of disks, the higher chance of failure. However, may throughput also be one limitation? Do you know anyone use disk array which contains large number of disks to handle small requests? Such as 256 disks to handle 4K requests? Thanks! Yuehai